Climate projections predict warming trends for the Canadian prairies. This study investigated effects of warmer spring temperatures on phytoplankton-zooplankton populations in small eutrophic lakes. A two-year study of three eutrophic lakes with contrasting spring weather conditions, i.e., 2005 - a 'normal' spring and 2006- a warm spring (+2oC), demonstrated that warmer water temperatures were associated with increased total phytoplankton and relative cyanobacteria biomass and a shift in zooplankton dominance from daphniids to rotifers. Zooplankton hatching experiments and computer simulations tested the hypothesis that a warm spring differently affected daphniid and rotifer emergence from resting eggs. Experimental conditions mimicking an earlier spring (shorter photoperiod) resulted in fewer daphniid but not rotifer hatchlings, and computer simulations indicated that these changes in hatching success could be responsible for shifts from daphniid- to rotifer-dominated systems. Overall, a warm spring negatively affected daphniid populations, indirectly by increasing cyanobacteria prevalence and directly by decreasing hatching success.